In the application of light-emitting diodes (LEDs), switch state control is widely applied to the control over many lamps. Compared with many kinds of intelligent light-regulation control, such control has the advantages of a simple system, low costs, strong substitutability, etc.
FIG. 1 is a schematic diagram of an existing switch light-regulation control system. With reference to FIG. 1, a switch light-regulation control system 100 commonly seen at present comprises an input rectifier 110, a light state regulation control circuit 120 and an output stage circuit 130. In the switch light-regulation control system 100 of FIG. 1, a diode D5, a resistor R1 and a capacitor C1 constitute a state sampling circuit for sampling a state of a switch before a rectifier bridge. A neutral line or live line voltage at a front end of the rectifier bridge is directly detected, and a state control signal is produced on a detection pin DET of a light state regulation controller 121 via two actions, i.e., turn-off and turn-on actions of a main loop switch (a wall switch) (SW). A discharge loop is integrated inside the light state regulation controller 121 to pull down the voltage at the DET pin when the switch is off, realizing state switching, thereby changing the state of an output load (i.e., an LED).
The advantages of such a light-regulation control system are having a fast response speed and being free from the impact of the output load. Once the switch (SW) is off, the isolated diode D5 immediately reverses, causing the voltage at the detection pin (DET) to decrease quickly, thus forming an effective state control signal, so that the Pulse Width Modulation (PWM) control state of the light state regulation controller 121 changes quickly, and thus the modulation of an output current of the LED is realized.
However, such a light-regulation control system still has its defects. Firstly, a component for sampling a switch state should be additionally added, leading to increased system costs; secondly, if an X capacitor C1 is added before the rectifier bridge, when the switch (SW) is off, a discharge time constant would increase due to a voltage keeping effect of the X capacitor, and the time period for forming an effective state control signal is extended, thereby easily causing a false action of the switch state.